Why synchros?
Aircraft constantly need to send an angle from one place to another: the position of a flap, trim tab or landing gear to the flight deck; heading from a compass system to several displays; a command angle to an antenna or autopilot. A synchro does it with nothing more than wires. It is an electromechanical rotary transformer whose output voltages describe its shaft angle, and a second synchro can turn those voltages back into an angle. Trade names you will meet include Selsyn, Autosyn (AC synchros) and Desynn (a DC system).
Synchro construction
- Rotor — a single winding on a salient-pole ("dumbbell") core, fed through slip rings with the AC reference (excitation): normally 26 V or 115 V, 400 Hz. Its leads are R1 and R2.
- Stator — three windings 120° apart, connected in star (Y), with leads S1, S2 and S3.
- The rotor induces a voltage in each stator winding whose size depends on the angle between the rotor and that winding. Together the three voltages are unique for every angle.
- The stator voltages are not three-phase: they are all in time phase (or antiphase) with the excitation; only their amplitudes change. The maximum line-to-line stator voltage is 11.8 V for a 26 V synchro and 90 V for a 115 V synchro.
- Electrical zero is the reference position: the rotor lies along the S2 winding and the S1–S3 voltage is zero. Synchros are "zeroed" to it when installed.
Torque synchro systems
In a torque system a torque transmitter (TX) is connected to a torque receiver (TR). Both rotors are fed in parallel from the same AC reference, and the stators are joined S1–S1, S2–S2, S3–S3. If both rotors are at the same angle, their stator voltages are equal and opposite and no current flows. If they differ, currents circulate in the stator wires, and the resulting magnetic field exerts a torque on the receiver rotor, turning it until it matches the transmitter.
- The torque is small, so a TR can only move a light load — an indicator pointer. Typical accuracy is around a degree.
- Receivers have a mechanical damper (often an inertia damper) so they do not oscillate or spin when power is applied or the transmitter moves suddenly.
- Classic uses: flap, trim and control-surface position indicators, and older engine instruments such as Autosyn oil-pressure and fuel-flow gauges.
Control synchro systems
When the load is heavy, or accuracy matters, the synchro is used as the error detector of a servo (Chapter 8). A control transmitter (CX) feeds a control transformer (CT). The CT's rotor is not connected to the supply. It picks up an error voltage proportional to (the sine of) the difference between its own angle and the transmitter's, and the error's phase shows the direction. A servo amplifier drives a servo motor, which turns the load and, through follow-up gearing, the CT rotor, until the error falls to null.
Control systems give plenty of torque and good accuracy. They drive compass-system heading outputs and slaved gyros, radar antenna positioning, autopilot servos and flight-director computations.
Differential synchros and resolvers
- A differential synchro — a torque differential transmitter (TDX) or control differential transmitter (CDX) — has three windings on both stator and rotor. Placed between a transmitter and a receiver, it adds its own shaft angle to, or subtracts it from, the transmitted angle. The receiver therefore shows θ1 ± θ2 — for example a radio magnetic indicator's needle combining heading and relative bearing, or a heading reference being slewed. A torque differential receiver shows the difference between two transmitters.
- A resolver has stator (and often rotor) windings set at 90° instead of 120°. With the rotor excited, its two outputs are proportional to sin θ and cos θ of the shaft angle. That makes it ideal for resolving a vector into components, and for converting between polar and rectangular co-ordinates, in navigation and attitude computers, inertial reference systems and electronic displays. Brushless resolvers also commutate electric motors.
DC selsyn and Magnesyn systems
- A DC selsyn (Desynn) runs on the 28 V DC bus. The transmitter is a circular resistance with two brushes, 180° apart, that turn with the shaft and carry the DC supply. Three taps 120° apart on the resistance connect to three coils in the indicator. As the brushes turn, the currents in the three coils change and the direction of their combined field rotates. The indicator's permanent-magnet rotor, carrying the pointer, aligns with that field. It is a simple system, used for older landing-gear, flap and trim position and fuel-quantity indicators.
- A Magnesyn is an inductance transmitter. In both transmitter and indicator, a permanent-magnet rotor sits inside a soft-iron toroidal (ring) coil with taps at 120°, and the coil is excited with AC (26 V, 400 Hz). The magnet's position sets the voltages at the three taps; the indicator's matching coil and magnet reproduce the angle. It was used for remote-reading compasses and engine pressure gauges.
- More generally, inductance transmitters are any sensors in which movement of a magnetic core changes inductance or coupling — the LVDT and RVDT of Chapter 8 and the E and I pick-off below are examples.
Capacitance transmitters
A capacitor's value depends on the material between its plates (its dielectric constant). The classic aircraft example is the capacitance fuel quantity system. Each tank holds several probes, each made of two concentric tubes. Air (K = 1) fills the gap above the fuel and fuel (K ≈ 2.1 for kerosene) fills it below, so the probe's capacitance rises with the fuel level.
- An AC bridge or digital electronics measures the capacitance. Summing several probes corrects for aircraft attitude.
- Because fuel's dielectric constant changes with its density, a compensator probe or densitometer lets the system indicate fuel mass (kg or lb) rather than volume.
- There are no moving parts. Water contamination (K ≈ 80) and dirty or damaged probes cause large errors, so probe cleanliness and wiring condition matter.
- The same principle gives capacitive pressure sensors (a diaphragm moving relative to a fixed plate) and capacitive proximity sensors.
B2 extension: E and I pick-offs, reversed leads and hunting
- An E and I transformer has an E-shaped laminated core with an AC primary on the centre leg and two secondaries on the outer legs in series opposition, plus a pivoted I-shaped armature. With the I bar central, the outputs cancel. When it tilts, the output's amplitude shows how far and its phase shows which way. It is used as a pick-off on gyros and in pressure sensors.
- Reversed synchro leads (a classic fault-finding question):
- Swapping S1 and S3: the receiver turns the wrong way, and reads correctly only at 0° and 180°.
- Swapping R1 and R2 on one unit: a constant 180° error.
- Moving all three stator leads one place round (S1→S2→S3→S1): a constant 120° error in the correct sense.
- Swapping S1–S2 or S2–S3: reversed rotation and an offset.
- Hunting in a synchro or servo system — oscillation about the correct reading — points to insufficient damping (for example a faulty receiver damper or rate feedback) or too much amplifier gain.